I too think that the attack happened either outside the flat, or inside with the door wide open. Otherwise, I don't think the sound of her screams or calls for help would have been heard as they were. I hadn't thought of her stepping out for the cat, but that could explain how he got in. Perhaps she came home and didn't immediately lock the door, set things down while the cat was out, cracked open a cider, turned on the oven and then opened the door to call the cat.
That's what I think as well. Probably in the hallway, is my feeling. I also think it happened very shortly after she got home.
Can anyone recall whether Greg said he had let the cat out or had left it in? I do remember about him leaving the light on for Joanna but not what he said about the cat.